1. Field of The Invention
The present invention relates to a new method for improvement of optical imaging. In particular the invention applies to the production of integrated circuits by projection lithography.
2. Description of the Related Art
A transparent mask is illuminated by a light source and the light transmitted by the mask is projected by a lens system to form an image on the wafer. The resolution of the system depends on the wavelength and on the numerical aperture of the lens. Present production of 16 megabyte dynamic RAM memories uses i-line (365 nm wavelength) illumination and a projecting lens with an NA of 0.40-0.5.
For the next generation 64 Mb DRAM, there are a number of options to increase resolution: shorter wavelength, higher NA, and so-called phase-shift masks. All of these are difficult to implement.
Shorter wavelength means excimer laser light source with increased operating costs, uncertain reliability and new resist processes. Higher NA requires flatter wafers, new resist technology (so-called top-surface imaging resist) and projection lenses with flatter field. Phase-shifting masks pose new problems since they are difficult to make and even worse to inspect and repair.